Fluid reasoning, or the capacity to think logically and solve problems in novel situations, is central to human cognition. The acquisition of fluid reasoning ability during childhood is thought to serve as a scaffold that supports learning in other cognitive domains, including reading and arithmetic. A fundamental question concerns the brain mechanisms that underlie the development of fluid reasoning over childhood and adolescence. The proposed research examines the typical developmental changes in brain structure and function associated with improvements in fluid reasoning between the ages of 5 and 17. An accelerated longitudinal design will be used, enabling the assessment of within-person changes over 1-3 years, with two measurement occasions per participant. Changes in brain structure will be assessed with structural magnetic resonance imaging and diffusion tensor imaging. Additionally, changes in brain function will be assessed with functional magnetic resonance imaging during the performance of two reasoning tasks. Finally, a battery of cognitive measures will be used to assess changes in reasoning ability, as well as processing speed, short-term memory, working memory, and executive function. Dynamical systems modeling will be used to examine the interrelations between brain structure, function, and performance from childhood to adolescence. These analyses will be used to evaluate hypotheses about the neural mechanisms underlying developmental changes in reasoning ability. This combined developmental, cognitive neuroscientific, and quantitative approach is novel, and should yield important advances on several fronts. First, the measurement of brain activation associated with fluid reasoning in individuals of varying age and ability level will provide fresh insights into the neural mechanisms underlying the changes in an important higher-level cognitive function about which relatively little is known. Second, up to now, the few published longitudinal studies on brain development have focused on changes in brain structure. As such, this longitudinal dataset will be invaluable in terms of characterizing typical developmental changes over a large part of childhood and adolescence in terms of both brain structure and function. Finally, this research may provide insights into the nature of reasoning deficits in a number of neurological disorders affecting children and/or adults, including Traumatic Brain Injury, autism, schizophrenia, and frontotemporal dementia.